It means- "Down To Fish" according to Vinny on the show Jersey Shore. Down To Fish can be used as an alternative if there's a girl wants you and truly doesn't know the meaning of "DTF" which truly means "Down To Fuck".

Hoe: Hey what does DTF stand for?
Vinny: Down To Fu....Down To Fish!
Hoe: Oh okay let's go fishing.
Vinny: I really would like to but I just ate a 3 dollar burrito and I really need to drop the kids off at the pool, sorry.